Field of the Invention
The invention relates to a method, a magnetic resonance apparatus, and a non-transitory, computer-readable data storage medium for reconstruction of image data from undersampled raw data and reference data.
Description of the Prior Art
In a magnetic resonance apparatus the object to be examined, for example a patient, is usually subjected by a basic field magnet to a relatively strong basic magnetic field, of 1.5 or 3 Tesla for example. In addition, a magnetic field gradient is applied by a gradient coil arrangement. Then, via a radio-frequency antenna, radio-frequency excitation signals (RF signals) are radiated by means of suitable antenna coils, which leads to the nuclear spins of specific atoms resonantly excited by this radio-frequency field being flipped by a defined flip angle in relation to the magnetic field lines of the basic magnetic field. In the resulting precession of the nuclear spin radio-frequency signals, so-called magnetic resonance signals (MR signals), are emitted, which are received by suitable reception antennas and then further processed. The data space in which the MR signals are present is referred to as k-space. The MR signals are digitized and stored in a k-space memory as a matrix of complex numerical values. The collection of complex numerical values of the k-space matrix is referred to as raw data. Associated image data are reconstructed from the raw data by execution of a multi-dimensional Fourier transformation, for example.
Since the image data can be reconstructed from the raw data by a Fourier transformation, the reconstruction is subject to the Nyquist criterion. The k-space data are transformed by the Fourier transformation into the image space. The k-space matrix is designed such that, when it is completely occupied by raw data (i.e. every available data entry point has been filled), the image data can be obtained from the raw data by an (if necessary) multi-dimensional Fourier transformation. The Nyquist criterion is then typically fulfilled. Consequently, if the k-space matrix is not completely occupied by raw data, then a Fourier transformation of the raw data results in erroneous image data.
For a particular measurement, a specific MR control sequence, also called a pulse sequence, is to be activated by the MR data acquisition scanner, which is composed of a sequence of radio-frequency pulses, in particular excitation pulses and refocusing pulses, as well as gradient pulses, suitably coordinated thereto. The gradient pulses create dynamic magnetic field gradients in different spatial directions, which are used for spatially encoding the raw data. The spatial encoding is typically done by a combination of different encoding methods. Phase encoding and frequency encoding are known examples of such encoding methods. Readout windows, which specify the periods of time in which the induced MR signals are acquired, must be set to match the encoding in time. In such cases the imaging is definitively determined by the timing within the sequence, i.e. the intervals in time at which pulses follow one another.
The application of an MR control sequence is typically more time-intensive as more raw data is recorded. A larger k-space matrix can be used, for example, for an increase in the resolution of the image data. In classical MR imaging, which employs a pure Fourier transformation for reconstruction of (preferably) error-free image data, a larger k-space matrix means a larger quantity of raw data, and thus an increase in the duration of the MR control sequence.
Parallel imaging is a widely-used method for shortening the measurement time and/or improving the resolution in MR imaging. The method is based on an undersampling of k-space, so that not all entries of the k-space matrix are occupied by raw data. The missing raw data for completing the k-space matrix can be provided, for example, as a result of symmetries or as a result of the knowledge of spatial sensitivity profiles of the receive antennas. Symmetries are utilized in the “Virtual Conjugate Coils” method (e.g. Blaimer et al., “Comparison of phase-constrained parallel MRI approaches: Analogies and differences”, doi 10.1002/mrm.25685, MRM) or in “Partial Fourier” reconstruction methods, such as “Phase Constrained” parallel imaging (e.g. Willig-Onwuachi et al., “Phase-Constrained Parallel MR Image Reconstruction: Using Symmetry to Increase Acceleration and Improve Image Quality”, Proc. ISMRM 2003, P. 19). Spatial sensitivity profiles of the receive antennas are used by SENSE or GRAPPA to speed up the recording, increase the resolution and/or reduce distortions.
Both image-based (e.g. SENSE) and k-space-based (e.g. GRAPPA) algorithms for parallel imaging require the recording of reference data, which allow deductions to be made about the sensitivity profile and/or the symmetry of the k-space data. In image-based methods, representative coil sensitivity profiles are recorded directly, and in k-space-based methods a representative section of k-space is completely sampled. The reference data will consequently be used to determine missing raw data in the k-space matrix. For reduction of artifacts in the image data, it is advantageous for the reference data and the raw data to have at least similar imaging characteristics. An imaging characteristic specifies how an external influence, such as an inhomogeneity of the basic magnetic field, influences the acquisition and/or the reconstruction of data. The imaging characteristic of an MR control sequence is determined, for example, by the sampling scheme used to enter data into k-space, i.e. by the order of the acquisition of the k-space data. Since the reference data typically are from an MR signal entered a, or at least one, position of k-space at which no raw data will be acquired, the sampling schemes for the recording of the raw data and the reference data differ. It is consequently advantageous to design the sampling schemes for the recording of the raw data and the reference data such that the two data sets will be similarly influenced by an external influence such as an inhomogeneity of the basic magnetic field, i.e. a frequency deviation. This characteristic is especially important when, in the reconstruction, in addition to the information contained in the reference data about the coil sensitivities, symmetry characteristics are also exploited, such as in the “Virtual Conjugate Coils” method mentioned above.
Distinct artifacts can occur in echo planar imaging (EPI) in combination with parallel imaging. EPI, by comparison with other MR control sequences, is especially dependent on frequency deviations (e.g. as a result of susceptibility-induced faults of the homogeneity of the basic magnetic field): Even a few tens of Hertz frequency deviation can lead to a shift of several pixels in the EPI image data, which in k-space typically corresponds to an overlaid phase variation. A frequency deviation is a typical external influence on an imaging characteristic of an MR control sequence. For example, if the reference data exhibit a lower sensitivity to frequency deviations than the raw data, then the result can be a smaller phase variation in the reference data, and the algorithm for parallel imaging is subject to the lower sensitivity to frequency deviations. This can lead to artifacts in the images during the reconstruction of the image data by an algorithm for parallel imaging.
The following methods for recording reference data are especially known for EPI.
The sampling scheme of EPI can be designed such that the segmentation of k-space, as used for the acquisition of the raw data, is transferred to the acquisition of the reference data. For example, if undersampled raw data are recorded for the parallel imaging with an acceleration factor of two, such that rows of the k-space matrix with even numbers are acquired while rows with odd numbers are left out, then this segmentation of k-space can be transferred to the acquisition of the reference data. The representative section of k-space to be recorded for the reference data can be segmented in a similar way, by a first block, containing rows with even numbers of the representative section of k-space to be recorded, serving as the reference data. The rows with odd numbers are acquired in a second block. An MR control sequence can be divided into a number of blocks, wherein the duration of a block typically corresponds to the repetition time that specifies the duration between two excitation pulses, which follows the acquisition of the same examination region of associated MR signals. After conclusion of the MR control sequence, the sum of all segments contains all reference data required for the reconstruction. The phase evolution of the reference data and of the raw data preferably matches. The advantage of this method is that with such a segmentation the reference data and the raw data exhibit the same imaging characteristics with respect to a frequency deviation, and thus no reconstruction artifacts occur. The disadvantage of this method is that it exhibits a high sensitivity to movement, since the recording of the reference data extends over several blocks due to the segmentation. The movement sensitivity specifies how sensitive the method is to a movement of the examination object, especially of the examination region.
In a further method for recording the reference data, the representative section of k-space is for the reference data is recorded by EPI after a single excitation, i.e. by one block of the MR control sequence. For example, all rows of the representative section of k-space are recorded in ascending order. Consequently there is no segmentation of the reference data analogous to the sampling scheme for the acquisition of the raw data. The raw data are accordingly not completely recorded in accordance with the sampling scheme's acceleration factor. The advantage of this method is that it exhibits a low sensitivity to movement because of the compact acquisition of the reference data. A disadvantage of the method is that the imaging characteristics of the reference data and of the raw data can be different because of the different segmentation, which can lead to reconstruction artifacts with external influences such as frequency deviation.
FLEET (“fast low-angle excitation echo-planar technique”, e.g. Polimeni et al., “Reducing Sensitivity Losses Due to Respiration and Motion in Accelerated Echo Planar Imaging by Reordering the Autocalibration Data Acquisition”, doi 10.1002/mrm.25628, MRM 2015) describes a further method for recording the reference data. The reference data are acquired segmented, preferably in a similar manner to the segmentation of the raw data, and the undersampled segments of the reference data are recorded at short intervals one after another by an echo planar sampling scheme. The sum of the segments contains all required reference data. Here the flip angle of the excitation pulses is modulated from segment-to-segment such that contrast variations between the segments are minimized. The fast recording of the reference data produces a low movement sensitivity of the method. Furthermore, during correct segmentation the reference data and the raw data exhibit the same imaging characteristics, which is why in this regard no reconstruction artifacts occur. However contrast variations between the segments can only be minimized for one type of tissue, which is defined by its T1- and T2-relaxation times. In tissues with strongly deviating relaxation times, segmentation artifacts occur, which lead to reconstruction errors within the framework of the parallel imaging.